Systems for monitoring force loads are well known and are used with a variety of products to determine structural conditions in mechanical components. These systems are characterized by a plurality of parameter sensors, such as strain gauges, which are positioned locally with the mechanical elements and provide signals to a remote processor. The measured data is correlated by the processor in accordance with a pre-established relationship. The processor output signals are indicative of the stress or loading conditions of the elements.
In many applications, the direct instrumentation of mechanical elements is possible. For example, strain gauges and sensors can easily be positioned locally on an airplane's wings and aerilons, with signals from the sensors conventionally routed to a remote cockpit processor. However, for some elements, such a rotating turbine blades in a jet engine, direct instrumentation is impossible or impractical. Another such example is a rotating helicopter blade. Direct instrumentation of the blade with locally positioned sensors can only be accomplished by employing a sophisticated slip ring apparatus which is prone to wear and must therefore be maintained on a frequency basis. As a result, remote blade mounted instrumentation is configured with a helicopter only for purposes of initial testing and calibration.
U.S. Pat. No. 4,485,678 to Fanuele discloses a Rotor Diagnostic and Balancing System which includes vibration and rotational sensors coupled to a processor providing outputs thereto. The processor is capable of generating a diagnostic output signal listing the origin of the vibration based upon preestablished standards and, if possible, effect a trim balance of the rotor. Those skilled in the art will note that the '678 system uses a maximum amplitude of the entire sensor signal and simple frequency domain analysis of the sensor system signals.
U.S. Pat. No. 4,764,882 to Braschel et al. discloses a method of monitoring fatigue of structural component parts of, for example, a nuclear power plant. The method is characterized by a plurality of sensors locally positioned on part (e.g. a feed water nozzle). On the basis of a local temperature distribution and/or the temperature versus time curve, the method will calculate respective temperature curves in the interior portion of the nozzle. A number of simplifications are necessary before a computed stress verses temperature curve can be generated. The '882 method uses direct sensing to a determine parameter magnitudes.
U.S. Pat. No. 4,345,472 to Hara et al. discloses a method and apparatus for digitally analyzing dynamic unbalance of a rotating body. A mechanical vibration of a rotating test body is fitted with a plurality of locally positioned sensors. The mechanical vibration is translated into a periodic signal which represents a combined vector of the unbalanced rotating body and the mechanism that rotates it. The '472 method divides the signal into two orthogonal vector components. Unknown values of four constants are determined by preliminary test having three successive stages involving rotating the test body a selected amount from a first stages position, sampling digital data signals and displacing the test body a predetermined amount with respect to that position. A trial dead weight of a known mass is mounted on the test body and it is rotated. In the third stage, a computer operates on the data and determines the four constants in accordance with a set of equations. Again, the '472 device directly senses a data signal and uses the entire data signal.
U.S. Pat. No. 4,758,964 discloses a method and apparatus for monitoring machine parts which is characterized by sensors that directly measure on the part the natural characteristic vibrational behavior thereof in operation and compares that measured vibrational signal to signals indicative of that part under normal operation. Should the signal differ substantially from the signal indicative of normal operation, an alarm is sounded. Note that the '964 apparatus uses direct sensing and uses the entire signal.
Techniques for indirectly measuring structural parameters of loaded mechanical elements have also been explored. With these techniques, sensors are remotely positioned from the loaded components of interest. For example, in an airplane, a plurality of sensors can be positioned on the aircraft body. The signal received at these sensors comprises a component due to the loading of the airplane body, as well as a component indicative of the loads on the aircraft wings. Using pre-determined relationships, a processor could ideally isolate the strain information from the element of interest and correlate that data to the loading of the element.
Ascertaining the signal component output loads on helicopter elements is inherently more difficult than for an airplane as the loads imposed upon the various rotating elements are periodic. Systems which have attempted to remotely determine structural parameters in the rotating elements of a helicopter, such as blade strains or moments, have been unsuccessful because the mathematical computations required have been deemed too complex and that the requisite data insufficiently measurable.
An example of a known system with remote sensing is disclosed and claimed in the commonly owned U.S. Pat. No. 4,894,787 and incorporated herein by reference. The '787 system is characterized by an apparatus mounted on the fixed system which receives signals from a rotating system. These signals are provided to a plurality of Fourier coefficient detectors to ascertain the respective Fourier coefficients therefor. Signals corresponding to these coefficients are combined with correlation coefficient signals previously determined. Sine and cosine generators provide signals to component synthesizers which also receive the combined coefficient output signals. The output therefrom is summed into a time dependent signal corresponding to the parameter of interest, such as helicopter blade bending moment.
The '787 system is burdened by the need to both decompose the sensed signal into its Fourier components and regenerate a time dependent signal using a multide of function generators. These requirements prevent the '787 system from being used in real time at high frequency and add undesirable complexity and cost. It would be advantageous to have a system for remote monitoring of parameters which would operate entirely in real time with a minimum of components. The present invention is directed towards such a system.